have to WORK for it. If you ARE a Legion Member, your epic helm is in another shop… Dage's Undead Legion Shop in Shadowfall.
Note: Also see Simple Skull Cap.
Thanks to Shadow dv, Shal, Tendou no Mazo and TreadLight.

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Must have completed the 'Hail to the King' quest to purchase.
If you bought the helm before the requirements were added to the shop, you now have the LegionWannabeHelm.
Thanks to Blaeku and Rezurrect.